Metalloenzymes often utilize a high‐valent metal‐oxo species in their catalytic cycle that is formed through a combination of reduction/proton transfer steps from O₂. In biomimetic chemistry, by contrast, the use of molecular oxygen is challenging and instead addition of iodosylarenes as terminal oxidants generates these high‐valent metal‐oxo intermediates. There has been controversy in the literature for many years, whether the iodosylarene (or one of its side‐products in a reaction with a metal center) can act as an active oxidant. Herein, we present a comprehensive theoretical study on the thioanisole oxidation by a range of possible oxidants originating from a reaction mixture of a Mn(III) complex with iodosylbenzene. We show here that actually there are multiple isomeric iodosylarene complexes in solution and a number of those react with substrates with rate‐determining barriers at par with high‐valent metal‐oxo species. In particular, the [Mn^III(PhIO)], [Mn^V(O)(PhIO)] and [Mn^IV(O)‐I(Ph)‐O^•] complexes are identified as competitive oxidants and as such care should be taken with interpreting experimental rate constants.